Circuit arrangement for the operation of wavelength division multiplexing

ABSTRACT

On an acousto-optical add/drop multiplexer to whose input the wavelength-division multiplex signal is applied, a plurality of surface acoustic waves are excited at specific frequencies and in each case a portion of the light power is separated from wavelength-division multiplex channels at an optical frequency which is governed by the frequency of such a surface acoustic wave; a superheterodyne receiver (HE), to whose input side the separated light power elements with one polarization are applied as well as light power elements with another polarization which are output from the non-separated elements of the wavelength-division multiplex signal emits an output signal which corresponds to the light power levels which are transmitted in a channel-specific manner in the individual wavelength-division multiplex channels, with whose aid a regulating device in each case separately readjusts the light power transmitted in the individual wavelength-division multiplex channels to a predetermined value.

BACKGROUND OF THE INVENTION

Future-proof optical telecommunications networks have to satisfystringent requirements relating to capacity and flexibility. Suchrequirements are optimally satisfied by transmission and switching usingoptical frequency-division multiplex (wavelength-division multiplex WDM)wavelength-division multiplex allows the capacity of opticaltransmission networks to be considerably increased; and WDM couplingarrangements (Optical Cross Connects OCC) allow the flexibility of suchnetworks to be enhanced. In order to achieve the high performance ofsuch networks with a complexity level which is as low as possible at thesame time, it should be possible to transmit the optical signals as faras possible without (electrooptical) regeneration. The use of fiberamplifiers allows the attenuation of optical signals on opticalconductors and in WDM switching matrices to be compensated for, so thatthe length of a section which is free of regenerators is in principlenot limited by attenuation in any case.

However, problems arise in wavelength-division multiplex systems fromthe fact that the individual WDM channels are attenuated or amplified todifferent extents. Minor differences are caused just by tolerances inthe individual system components (such as fiber amplifiers, opticalfibers, connectors, WDM switching matrices); and in the case of longsections without regenerators, these can accumulate so as to result inlevel differences occurring which prevent clean separation of thewavelength-division multiplex channels. It is thus desirable to be ableto insert into the network components which reduce these leveldifferences.

At a normal ambient temperature (owing to homogeneous linear propagationat a normal ambient temperature), such level differences cannot bereduced by means of a fiber amplifier. It is admittedly known that fiberamplifiers have non-homogeneous linear propagation at low temperatures(77 K) (IEEE Photonics Technology Letters, 2 (1990), p. 246-248;OFC/IOOC'93 Technical Digest, p. 174-175); however, the amount ofcooling required in this case prevents practical use.

In principle, it is known (from IEEE Photonics Technology Letters, 8(1994) 11, p. 1321-1323) for level regulation to be carried out byphysically separating the individual WDM channels from one another usinga WDM demultiplexer, and for the optical signals in each channel to beamplified in their own right by means of a channel-specific fiberamplifier operated in the saturation region, after which the signals areonce again combined in wavelength-division multiplex form by means of aWDM multiplexer. Level regulation for a fixed wavelength scheme can thusbe achieved by using channel-specific optical components, acorrespondingly large number of which therefore have to be provided.

In contrast, the invention indicates a different way to achieve levelregulation.

SUMMARY OF THE INVENTION

The invention relates to a circuit arrangement for the operation of awavelength-division multiplex system; this circuit arrangement ischaracterized, according to the invention, in that, on anacousto-optical add/drop multiplexer to whose input thewavelength-division multiplex signal is applied, a plurality of surfaceacoustic waves are excited at specific, different frequencies and aportion of the light power is separated from wavelength channels at anoptical frequency which is governed by the frequency of such a surfaceacoustic wave, and in that a superheterodyne receiver is provided towhose input side the separated light power elements are applied as wellas light power elements which are output from the non-separated elementsof the wavelength-division multiplex signal in a directional couplerwhich is connected downstream of the acousto-optical add/drop.multiplexer, and which superheterodyne receiver emits an output signalwhich corresponds to the light power levels which are transmitted in achannel-specific manner in the individual wavelength channels.

Such a circuit arrangement provides the capability for a deliberatedetermination, which can be flexibly matched to a wavelength scheme, ofthe optical levels which occur in each case in a plurality of wavelengthchannels, using only one acousto-optical add/drop multiplexer. In afurther refinement of the invention for this purpose, it is possible toprovide electrical mixing of the electrical signals which excite thesurface acoustic waves at different frequency and the electrical outputsignal of the superheterodyne receiver in order to use the output signalof the superheterodyne receiver to obtain the channel-specificindividual signals which correspond to the light power levels which aretransmitted in a channel-specific manner in the individual wavelengthchannels.

If, according to a further invention, channel-specific control of theintensity—which governs the proportion of the separated light power inthe individual wavelength channels—of the surface acoustic waves isprovided, then this advantageously—once again using only oneacousto-optical add/drop multiplexer—also allows channel-specificattenuation of the individual wavelength channels, in which case therelevant WDM channels are determined on the basis of the respectiveoptical wavelength, in accordance with the frequency of the acousticwaves.

In order to make it possible to avoid undesirable level differencesthroughout the entire WDM system, it is possible, in a furtherrefinement of the invention, to connect downstream from thesuperheterodyne receiver a regulating device for in each case separatelyreadjusting the light power transmitted in the individual wavelengthchannels to a value predetermined for the respective wavelength channel.In this case, in a further refinement of the invention, it is possible,by electrically mixing the electrical signals which excite the surfaceacoustic waves at a different frequency and the electrical output signalof the superheterodyne receiver, for the regulating device to separatefrom one another the signals which are contained therein and correspondto the light power transmitted in the individual wavelength channels andthus, in a channel-specific manner, to control the intensity of theelectrical signals which excite the surface acoustic waves.

In conjunction with a regulated fiber amplifier (or else a plurality ofregulated fiber amplifiers), this allows there to be long distancesbetween (electrooptical) regenerators in the wavelength-divisionmultiplex system.

At least the purely optical components of the superheterodyne receivercan, according to a further irvention, be integrated on a substratetogether with the acousto-optical add/drop multiplexer, whichadvantageously makes it possible to provide the optical functions oflevel measurement and level adjustment using a single acousto-opticalcomponent. The invention advantageously requires no optical componentsto customize the individual wavelength channels.

Further special features of the invention will become evident from thefollowing more detailed explanation of the invention, with reference tothe drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the block diagram of an exemplary embodiment of a circuitarrangement according to the invention; and

FIG. 2 shows circuitry details of a regulating circuit which can be usedin this circuit arrangement.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the circuit arrangement which is illustrated schematically in FIG. 1,to an extent required to understand the invention, for the operation ofwavelength-division multiplex systems, an input fiber F_(a) whichcarries the wavelength-division multiplex signal leads to anacousto-optical add/drop multiplexer ADM which is formed by apolarization splitter PS, two acousto-optical polarization convertersAOPC which act oppositely to one another, and a polarization combinerPC. Such acousto-optical add/drop multiplexers are known per se (forexample from Proc. VI^(th) European Conference on Integrated Optics(April 1993), 10-1 . . . 10-3) so that, to this extent, there is no needfor any further explanations here. A regulating device RE by an outputre excites a plurality of surface acoustic waves at specific, differentfrequencies f_(i) on the acousto-optical add/drop multiplexer ADM, withthe consequence that part of the light power is in each case separatedfrom wavelength channels at an optical frequency which is governed bythe frequency f_(i) of such a surface acoustic wave, that is to say thewavelength-division multiplex signal from the acousto-optical add/dropmultiplexer ADM is not output via said acousto-optical add/dropmultiplexer ADM in the direction of the output fiber F′ which continuestowards the output fiber F_(a) of the circuit arrangement, but via theoutput fiber F″. The light power which continues in the relevantwavelength channel via the output fiber F′ is thus correspondinglyreduced in comparison with the light power supplied via the input fiberF_(a).

An optical directional coupler RK is inserted between theacousto-optical add/drop multiplexer ADM and the output fiber F_(a) ofthe circuit arrangement in which a portion of the light power which ispassed on, that is to say of the non-separated elements of thewavelength-division multiplex signal, is output towards a fiber F″′.This fiber F″′, which is expediently a polarization-maintaining fiber inthe same way as the fiber F″, leads together with the fiber F″ to thetwo inputs of a superheterodyne receiver HE. In this case, appropriatefiber rotation is used to take account of the fact that the element ofthe light power which continues via the output fiber F′ has a differentpolarization (differing by 90°) from that of the light power elementwhich is output via the output fiber F″, but that the light signalswhich are supplied to the two inputs of the superheterodyne receiver HEwhich is illustrated in the exemplary embodiment according to FIG. 1must have the same polarization. As can also be seen from FIG. 1, such asuperheterodyne receiver can be formed with an optical (for example3-dB) coupler K and two photodiodes PD; an electrical amplifier V can beconnected downstream from it, as can likewise be seen from FIG. 1. Inthis case, although there is no need to illustrate this in more detailin FIG. 1, the purely optical components of the superheterodyne receiverHE can be integrated on a substrate together with the acousto-opticaladd/drop multiplexer ADM as well as the directional coupler RK.

At the output he, the superheterodyne receiver HE emits an output signalwhich corresponds to the light power levels transmitted in achannel-specific manner in the individual wavelength channels. Thissignal is supplied to the regulating device RE which, for its part andas has already been stated, excites a plurality of surface acousticwaves at specific, different frequencies f_(i) on the acousto-opticaladd/drop multiplexer ADM.

An acousto-optical interaction between the optical wave of a WDM channeli with essentially one, and only one, acoustic wave at a specificfrequency fi takes place on the acousto-optical add/drop multiplexerADM. The optical frequency of the light which is separated in the caseof this WDM channel i via the output fiber F″ of the acousto-opticaladd/drop multiplexer ADM is increased by exactly this frequency f_(i) incomparison with the frequency of the light which continues on the outputfiber F_(a) of the circuit arrangement. The output signal (he) of thesuperheterodyne receiver HE thus contains a spectral component at thefrequency f_(i), whose amplitude is a function of the power level of theWDM channel i. Based on a plurality of excited acoustic waves atdifferent frequencies f_(i), the light signal levels in each of aplurality of optical WDM channels i can be regulated separately, withthe corresponding acoustic waves being excited with the respectivelyrequired intensity.

In order to use the output signal of the superheterodyne receiver HE toobtain channel-specific individual signals which correspond to the lightpower levels transmitted in a channel-specific manner in the individualwavelength channels, it is possible to provide electrical mixing of theelectrical signals which excite the surface acoustic waves at adifferent frequency and the electrical output signal of thesuperheterodyne receiver HE. To this end, as is also sketched in FIG. 2,the output signal of the superheterodyne receiver HE can be multipliedby the signal which is supplied frog a corresponding signal generatorand is at the relevant frequency f_(i) as well as by the signal delayedby 90° (quadrature signal) at the relevant frequency f_(i), and theproducts I_(i) and Q_(i) can in each case be squared and then added andlow-pass filtered to produce a mixed signal m_(i) which is proportionalto the square (|a_(i)|²) of the channel-specific light signal amplitudea_(i); the ratio $\frac{m_{i}}{k_{i}\eta_{RK}},$

divided by the output level η_(RK) of the directional coupler (RK), ofthis mixed signal m_(i) to a light signal level k_(i) which is desiredon the output fiber F_(a) of the circuit arrangement is used, withc₁=arctan $( \frac{m_{1}^{a}}{k_{1}\eta_{RK}} ),$

to obtain a control signal c_(i) for amplitude regulation of theelectrical signal which excites the surface acoustic wave at thecorresponding frequency f_(i) on the acousto-optical add/dropmultiplexer ADM.

The regulating device RE (see also FIG. 1) according to FIG. 2 contains,per excitation frequency (f_(i)), two multiplication circuits MI_(i),MQ_(i) for multiplication of the output signal of the superheterodynereceiver HE (in FIG. 1) by the signals supplied from the individualexcitation signal generators . . . , Gf_(i), . . . at the individualexcitation frequencies (f_(i)) and by the excitation signals delayed by90° (quadrature signals). The outputs I_(i), Q_(i) of the twomultiplication circuits MI_(i), MQ_(i) respectively lead to the twoinputs of a squaring and addition circuit I_(i) ²+Q_(i) ², downstreamfrom which a low-pass filter TP_(i) is connected. The filter outputsignal (m_(i)) of the low-pass filter TP_(i) is proportional to thesquare (|a_(i)|²) of the channel-specific light signal amplitude (a_(i))and is supplied to a non-linear signal former c_(i) which is connecteddownstream from this low-pass filter TP_(i) and, in accordance with theratio $\frac{m_{i}}{k_{i}\eta_{RK}},$

divided by the output level η_(RK) of the directional coupler RK (inFIG. 1), of this filter output signal (m_(i)) to the light signal level(k_(i)) desired on the output fiber F_(a) (in FIG. 1), where c₁=arctan$( \frac{m_{1}}{k_{1}\eta_{RK}} ),$

forms a control signal for amplitude regulation of the excitation signalfor the corresponding frequency (f_(i)).

Controlling the amplitude of the electrical signal which excites asurface acoustic wave at a frequency f_(i) on the acousto-opticaladd/drop multiplexer ADM results in the intensity—which governs theproportion of the separated light power of the wavelength channelcorresponding to the frequency f_(i)—of the surface acoustic wave beingcontrolled in a corresponding manner, and thus allows channel-specificattenuation of the level of the light signal power of a WDM channel. Thelight power level transmitted in the individual WDM channels can in eachcase be readjusted separately to a value specified for the respectiveWDM channel, so that the output power of the WDM channels can also ineach case be set separately to a common, constant level. The input powerlevel of the wavelength channels may in this case fluctuate in a rangewhich is limited essentially by the characteristics of theacousto-optical add/drop multiplexer ADM.

What is claimed is:
 1. A circuit arrangement for the operation ofwavelength-division multiplex systems, characterized in that, on anacousto-optical add/drop multiplexer (ADM) to whose input awavelength-division multiplex signal is applied, a plurality of surfaceacoustic waves are excited at specific, different frequencies and aportion of a light power is separated from wavelength channels at anoptical frequency which is governed by the frequency of such a surfaceacoustic wave, and in that a superheterodyne receiver (HE) is providedto whose input side separated light power elements are applied as wellas light power elements which are output from non-separated elements ofthe wavelength-division multiplex signal in a directional coupler (RK)which is connected downstream of the acousto-optical add/dropmultiplexer (ADM), and which superheterodyne receiver (HE) emits anoutput signal which corresponds to light power levels which aretransmitted in a channel-specific manner in the individual wavelengthchannels.
 2. The circuit arrangement as claimed in claim 1, which asmeans to create an electrical mixture of electrical signals which excitethe surface acoustic waves at a different frequency, and the electricaloutput signal of the superheterodyne receiver (HE).
 3. The circuitarrangement as claimed in claim 1, wherein the surface acoustic waveshaving an intensity which is controlled on a channel-specific basis. 4.The circuit arrangement as claimed in claim 3, wherein in that thesuperheterodyne receiver (HE) has connected downstream of it aregulating device (RE) for in each case separately readjusting the lightpower transmitted in the individual wavelength channels to a value whichis predetermined for the respective wavelength channel.
 5. The circuitarrangement as claimed in claim 4, wherein by electrically mixingelectrical signals which excite the surface acoustic waves at adifferent frequency and the electrical output signal of thesuperheterodyne receiver (HE), the regulating device (RE) separates fromone another the signals which are contained therein and correspond tothe light power transmitted in the individual wavelength channels, and,in a channel-specific manner, controls the intensity of the electricalsignals which excite the surface acoustic wave.
 6. The circuitarrangement as claimed in claim 5, wherein two multiplication circuitsfor multiplication of the output signal of the superheterodyne receiver(HE) by signals which are supplied from individual excitation signalgenerators at individual excitation frequencies (f_(i)) as well asexcitation signals delayed by 90° are provided per excitation frequency(f_(i)), in that each of the outputs (I_(i), Q_(i)) of the twomultiplication circuits leads to two inputs of a squaring and additioncircuit (I_(i) ²+Q_(i)) downstream from which a low-pass filter isconnected, and in that a filter output signal (m_(i)) of the respectivelow-pass filter is proportional to a square (|a_(i)|²) of achannel-specific light signal amplitude (a_(i)) and is supplied to anon-linear signal former (c_(i)) which is connected downstream from thislow-pass filter and, in accordance with a ratio$\frac{m_{1}}{k_{1}\eta_{RK}},$

divided by an output level η_(RK) of the directional coupler (RK), ofthis filter output signal (m_(i)) to a light signal level (k_(i))desired on an output fiber (F_(a)), where C₁=arctan$( \frac{m_{1}}{k_{1}\eta_{RK}} ),$

forms a control signal for amplitude regulation of the excitation signalfor the corresponding frequency (f_(i)).
 7. The circuit arrangement asclaimed in claims 4, wherein the purely optical components of thesuperheterodyne receiver (HE) are integrated on one substrate togetherwith the acousto-optical add/drop multiplexer (ADM).
 8. A circuitarrangement according to claim 1, wherein purely optical components ofthe superheterodyne receiver are integrated on one substrate togetherwith the acousto-optical add/drop multiplexer.